What size inverter do I need for a battery backup?

The starting point is a load list. Add up the running watts of everything you want to keep on, then add around 15 to 20 per cent headroom for inverter losses, standby consumption and real-world variation¹. Battery capacity in kilowatt hours is total device power in watts multiplied by run time in hours². For planning, multiply battery energy by about 0.85 to 0.9 to get usable capacity, then divide by average load³.

Victron Energy, the Dutch manufacturer behind much of the UK's off-grid and backup equipment, states a 5 year warranty on its energy storage systems, upgradable to 10 years by an additional 10 per cent of the initial investment⁴. Its equipment is sold in the UK through specialist suppliers such as Wind & Sun⁵.

What an inverter does in a battery backup system

A battery inverter converts battery DC power to AC power for use by loads or the grid, and may also convert AC power to DC to charge the battery⁸. That dual role is what separates a backup inverter from a simple power supply: it sits between the bank and the house, shaping the current in both directions.

A backup-capable setup typically includes battery storage, a compatible hybrid inverter, control functions for safe off-grid switching, and a design that prioritises critical loads⁹. Backup design depends on the inverter, transfer equipment, battery capacity, household loads, and how the system is configured¹⁰. The inverter is therefore one component in a chain, and its rating has to match the rest of it.

In an energy storage inverter, a battery reserve function manages the battery's state of charge to ensure it remains within the desired range¹¹. That reserve is what stops a backup system draining itself flat on ordinary self-consumption and then having nothing left when the grid fails. Some systems integrate the inverter and battery management in a single unit with backup capability built in¹².

For a household, the inverter is the point at which stored DC energy becomes usable mains electricity. It is also the point of dependence: the inverter is a manufactured device with a warranty, a firmware stack and, in many designs, a communications link to a monitoring app or portal. Independence from the grid does not mean independence from the maker.

Sizing the inverter: matching continuous and peak power to your loads

The continuous rating must cover the sum of the loads running at once. The peak rating must cover the surge when a fridge compressor, pump or motor starts. A Full Backup option is stated as covering both smaller single-phase and larger loads like electric cookers or heat pumps⁹, which shows the range a single inverter may be asked to serve.

Work from a written list. Battery capacity in kilowatt hours equals total device power in watts multiplied by run time in hours². Add around 15 to 20 per cent headroom for inverter losses, standby consumption and real-world variation¹. For planning, multiply battery energy by about 0.85 to 0.9, then divide by average load³. That gives the run time a given bank can support at a given draw.

Battery storage should then be sized with a usable capacity of approximately 1 to 1.5 times the daily PV output¹³. That rule links the bank to generation rather than to load, and the two approaches need reconciling: a bank sized for overnight self-consumption may be larger or smaller than one sized for a three-day outage.

AC-coupled battery storage can be scaled by adding multiple energy storage units or microinverters as needed, with battery storage and system output power both growing¹⁴. That matters for sizing: a modular design lets the inverter rating and the bank grow together rather than forcing one oversized choice at the outset.

Victron Energy: the Dutch maker behind many UK backup inverters

Victron Energy B.V. is named as the manufacturer on its own declarations and certificates¹⁵. The company states over 45 years of experience building backup and off-grid systems⁷, and separately describes itself as a family run business with over 50 years of experience⁴. The two figures appear in different documents and are not reconciled here.

Victron offers a wide range of advanced batteries designed for energy storage in both residential and professional applications, providing reliable power for solar, backup, and off-grid systems¹². Its VE.Direct inverters use screw terminals for the battery connection and carry a VE.Direct communication port¹⁴. The MultiPlus range is described as a powerful true sine wave inverter and a sophisticated battery charger with adaptive charge technology¹¹.

The company's off-grid range supports deep integration with third party managed battery brands via GX devices, and AC-PV inverters via a software assistant⁷. Managed batteries can be integrated through GX devices in minutes⁴. AC-PV inverters are supported directly after installing a software assistant, with strong collaboration with Fronius PV inverters⁴.

For a household, this matters in two directions. A Victron-based system can be assembled from a wide range of batteries and PV equipment rather than a single maker's catalogue, which supports independence from any one supplier. Against that, the system's intelligence sits in Victron's GX devices and firmware, so configuration, updates and fault-finding run through the maker's ecosystem and its dealer network.

Warranty: 5 years standard, upgradable to 10

Victron Energy states a 5 year warranty on its energy storage systems, and that the standard 5 year warranty is upgradable to 10 years by an additional 10 per cent of the initial investment⁴. The upgrade is a paid extension rather than an automatic entitlement, and the cost is expressed as a proportion of what was spent rather than a fixed sum.

Other warranty terms in the market vary widely, which is useful context when comparing. One hybrid inverter model carries a 5 year warranty with a 10 year option¹⁷. Independent guidance states that it is standard for inverters to have warranties for 10 years¹⁸, while the Energy Saving Trust says most inverters have warranties of five years as a minimum, which you can often extend by up to 15 years¹⁹. A separate installer source describes a 5 year defects guarantee extendable to 10, 15, 20 or 25 years at additional cost²⁰.

Victron's battery ranges carry different terms from its inverters. The VRLA GEL battery range comes with a 2 year world-wide warranty, and the VRLA AGM range is compliant with both CE and UL specifications in ABS fireproof containers²¹.

Buying and support in the UK

Victron equipment reaches UK households through distributors and specialist suppliers. Wind & Sun lists the brand within its battery charger collection⁵, and its Victron ESS package is described as suitable for use with any PV system and as an ideal self-consumption system to add battery storage to new or existing solar PV systems²². Prices for this class of equipment are installer-quoted rather than published as a single retail figure.

Victron Energy states that it offers full technical support in the UK²³. That sits behind the dealer and installer network, which handles commissioning, GX configuration and battery integration. For a household, the practical support route is the UK supplier or installer who designed the system.

Three-phase solutions are available for larger properties. Victron and Sigenergy both offer genuine three-phase battery solutions which can be designed and installed for commercial or larger residential properties with a three-phase supply²³. Most homes have a single-phase supply, so this applies mainly to larger or commercial premises.

The wider context is a grid-scale storage build-out: UK grid-scale battery storage power capacity reached 7.5 GW by the end of 2025²⁴, with a projection of 50 GW by 2035²⁵. That is grid infrastructure rather than household equipment, but it shapes the market for cells, inverters and installation labour that home systems draw on.

Integrating batteries and solar PV with a Victron system

Victron systems support PV in parallel, AC-coupled PV, DC-coupled PV or a combination of all these options⁴. That flexibility is the main argument for the architecture: a household can add generation in the form it already has, rather than replacing it.

Managed batteries can be integrated through GX devices in minutes, and AC-PV inverters are supported directly after installing a software assistant⁴. The off-grid range extends this to deep integration with third party managed battery brands via GX devices and AC-PV inverters via a software assistant⁷. The Victron ESS battery storage add-on is stated as suitable for use with any PV system²².

Adding generation later is a normal path. The Energy Saving Trust advises considering battery storage if a solar system does not already have it, to store surplus electricity instead of exporting it²⁶. It also notes that if you have battery storage, you can store excess electricity from wind turbines and solar panels to use later⁶.

The economics need stating plainly. Adding a battery to a solar panel system can increase bill savings, but often not enough to recover the cost of the battery within its expected lifetime, particularly where households already have access to a good export tariff²⁶. A battery bought for backup resilience is a different proposition from one bought purely to shift cheap units, and the two cases should be assessed separately.

For independence, the picture is mixed. A Victron system with a managed battery and PV can run essential loads through a grid outage and store its own generation, which is real autonomy at the circuit level. What remains is dependence on the inverter's firmware, the GX device, the monitoring platform and the dealer who configured it, plus the battery maker's own management system.